6.1.1 This chapter provides an assessment of the potential water quality impacts associated with the construction of the proposed development. The purpose here will be to identify the discharges that may cause adverse impacts on identified water sensitive receivers if allowed to enter them uncontrolled. Mitigation measures will be proposed, if required, to ensure that the residual impacts comply with the relevant Legislation, Guidelines and Technical Memoranda.
6.2.1 The subject site is located at the Western end of New Territories and falls within the North Western Water Control Zone in accordance with the Water Pollution Control Ordinance Cap. 358. Therefore, Water Quality Objectives (WQOs) for this Water Control Zone will provide the criteria for this assessment. The appropriate WQOs are tabulated below1:
Table 6.1 Water Quality Objectives – North Western Water Control Zone
|
Parameter |
Water Quality
Objective |
Part or Parts
of Zone |
|
Dissolved Oxygen |
(a) Waste
discharges shall not cause the level of dissolved oxygen to fall below 4 mg/l
for 90 % of the sampling occasions during the whole year. In addition, the
concentration of dissolved oxygen should not be less than 2 mg/l within 2 m
of the seabed for 90 % of the sampling occasions during the whole year. |
Marine Waters |
|
|
(b) Waste
discharges shall not cause the level of dissolved oxygen to be less than 4
mg/l. |
Tuen Mun (A), (B) and (C) Sub-zones, Water Gathering Ground Sub-zones
and other inland waters |
|
pH |
(a) The pH of the
water should be within the range of 6.5-8.5 units. In addition, waste
discharges shall not cause the natural pH to be extended by more than 0.2
units. |
Marine waters excepting Bathing Beach Sub-zones |
|
|
(b) Waste
discharges shall not cause the pH of the water to exceed the range of 6.5-8.5
units. |
Tuen Mun (A), (B) and (C) Sub-zones, Water Gathering Ground Sub-zones |
|
|
(c) The pH of the
water should be within the range of 6.0-9.0 units. |
Other inland waters |
|
|
(d) The pH of the
water should be within the range of 6.0-9.0 units for 95% of samples
collected during the whole year. In addition, waste discharges shall not
cause the natural pH range to be extended by more than 0.5 units. |
Bathing Beach Sub-zones |
|
Suspended Solids |
(a) Waste
discharges shall neither cause the natural ambient level to be raised by more
than 30 % nor give rise to accumulation of suspended solids, which may
adversely affect aquatic communities. |
Marine Waters |
|
|
(b) Waste
discharges shall not cause the annual median of suspended solids to exceed 20
mg/l. |
Tuen Mun (A), (B) and (C) Sub-zones, Water Gathering Ground Sub-zones |
|
|
(c) Waste
discharges shall not cause the annual median of suspended solids to exceed 25
mg/l. |
Other inland waters |
|
Ammonia |
The unionised
ammoniacal nitrogen level should not be more than 0.021 mg/l calculated as
the annual average. |
Whole zone |
|
Nutrients |
(a) Nutrients shall
not be present in quantities sufficient to cause excessive or nuisance growth
of algae or other aquatic plants. |
Marine waters |
|
|
(b) Without
limiting the generality of objective (a) above, the level of inorganic
nitrogen should not exceed 0.3 mg/l expressed as annual water column average. |
Castle Peak Bay Sub-zone |
|
|
(c) Without
limiting the generality of objective (a) above, the level of inorganic
nitrogen should not exceed 0.5 mg/l expressed as annual water column average. |
Marine waters excepting Castle Peak Bay Sub-zone |
6.2.2 The Technical Memorandum – Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters2 states the limits that would make effluents acceptable for discharge into foul sewers, storm drains, inland and coastal waters. Thus wastewater leaving the site should conform to the Technical Memorandum.
6.3 Water Sensitive Receivers and Baseline Conditions
6.3.1 The subject site is located near the nullah opening at the northern part of Tuen Mun. It is bounded to the east by the waterfront of the typhoon shelter at Castle Peak Bay. The ambient water quality data was obtained from the EPD’s web site. A review of the data shows that:
Table 6.2 Ambient Water Quality
|
Parameter |
Concentration* |
|
Dissolved
Oxygen |
5.3 (3.6-6.7) |
|
pH |
8 (7.8-8.2) |
|
Suspended Solids |
18.3 (8.0-40.0) |
|
Total Kjeldahl
Nitrogen |
1.06 (0.38-1.16) |
|
Ammoniacal
Nitrogen |
0.26 (0.09-0.56) |
*All values except pH are in mg/l.
6.4.1 The assessment of water quality impact was conducted by following the items given as under:
· Identification of wastewater generating activities for the construction phase
· Identification of wastewater generating activities for the operation phase
· Assessment of potential impacts of the wastewater
· Assessment of the adequacy of the sewer and sewage treatment plant capacities
· Recommendation of mitigation measures if required
6.5 Construction Phase Impacts
6.5.1 The construction work can be divided into two phases viz. foundation and superstructure. The foundation construction is anticipated to last for 9 months and the superstructure construction for 17 months. Bored pile foundation will be provided. As advised by the ASD, the maximum amount of wastewater discharged during construction is estimated to be not more than 10 m3/d during the peak period i.e. foundation construction. Since the subject is served by a minimum 300 mm diameter sewer, it is anticipated that the sewer is capable to serve the small amount of wastewater generated from the construction activities. The wastewater generated will mainly be laden with suspended solids (silt, mud).
In addition to the activities mentioned above, the water quality will also be affected by the following:
· wash water from vehicles and equipment and dust suppression
· sewage generated due to site workers
· occasional oil and grease spillage from vehicles and plants
· run-off from site surfaces, material stock piles and drainage channels
6.6.1 The population of accommodation has been extracted from Table 2.1 and is presented below in Table 6.3.
Table 6.3 Population of Accommodation of Proposed Facilities
|
Proposed Facilities |
Population of Accommodation |
|
Wholesale Marine Fish Market |
440 persons |
|
Community Hall |
500 persons |
|
Other Possible Community Uses |
400 persons |
|
Refuse Collection Point (RCP) / Marine Refuse Collection Point (MRCP) |
10 persons |
|
Marine Park Management Office |
10 persons |
|
Spectator Stand* |
1000 persons |
* Only on the day
of Dragon Boat Festival
6.6.2 For the sewage impact assessment the population from the Spectator Stand has been excluded since this is the additional population expected on the day of the Dragon Boat Festival. The population for the Dragon Boat Festival is basically of a transient nature, therefore temporary chemical toilets will be provided on this day to cope with the additional sewage.
6.6.3 Based on information provided by the Agriculture, Fisheries and Conservation Department, and the flow estimation calculations in accordance with the Drainage Service Departments Sewerage Manual Part 1 the maximum daily amount of wastewater generated from the fish market is expected to be:
Fish Market sewage generation = 46.2 m3 per day
Daily market floor cleaning (using seawater) = 24.3 m3 per day
Daily seafood cleaning (using seawater) = 3.5 m3 per day
Community Hall = 75.0 m3 per day
Other Possible Community Uses = 60.0 m3 per day
Marine Park Management Office = 3.6 m3 per day
RCP & MRCP = 2.9 m3 per day
Total = 216 m3 per day
6.6.4
The
above calculations assume the following:
-
The
average fish market sewage generation is 7.7 m3/day (provided by
AFCD) and a peaking factor of 6 was assumed to obtain the maximum daily flow.
-
For
the Community Hall and other possible community uses a Global Unit Flow Factor
of 0.025 m3/d/head (for institutions) and a Peaking Factor of 6 were
assumed.
-
For
the Marine Park Management Office a Global Unit Flow Factor of 0.060 m3/d/head
(for employed population) and a Peaking Factor of 6 were assumed.
-
The
above Global Unit Flow Factors and global Peaking Factors were taken from
Tables 2 & 3 in the Sewerage Manual Part 1, DSD.
-
For
the RCP & MRCP it was assumed that there are 2 water taps, operated twice a
day for half an hour each, with a daily flowrate of 8.64m3/day per
tap.
6.6.5 As per information provided by the Drainage Services Department, Mainland North, the area is served by a minimum 300 mm diameter pipe. It is estimated that this pipe will have a mean flow velocity of 0.072m3sec, assuming a Mannings roughness coefficient of 0.015 for a concrete pipe (Table 6 in Sewerage Manual Part 1, DSD). It is anticipated that the 216 m3 per day of sewage generated from the proposed Joint User Complex and Wholesale Fish Market will amount to approximately 7% of the existing foul sewer capacity. Therefore, it is anticipated that there will be adequate capacity in the existing foul sewer and there will not be any adverse sewage impact.
6.6.6 The wastewater characteristics for the flow generated from the activities at the WFM and from the other Joint Users would be different. Whereas the wastewater from the other Joint Users will be similar to domestic sewage, the WFM wastewater is likely to be more polluted with organic matter. Table 6.4 shows the wastewater characteristics of the wastewater generated from the other Joint Users (typical) and from the WFM (measured).
Table 6.4 Characteristics of Wastewater
|
Parameter |
Concentration* |
|
|
|
Other Joint Users |
WFM |
|
PH |
6.5 |
8 |
|
Biochemical Oxygen Demand, BOD |
200 |
420 |
*all values
except pH are in mg/l
6.6.7 The above values are acceptable according to the Technical Memorandum – Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters, effluents discharged into foul sewers leading to Government sewage treatment plants (Table 6.5).
Table 6.5 Standards for Effluents Discharged into Foul Sewers
|
Parameter |
Concentration* |
|
pH |
6-10 |
|
Biochemical Oxygen Demand, BOD |
1000 |
*all values
except pH are in mg/l
6.6.8 Confirmation was also obtained from the Drainage Services Department, Sewage Treatment Division 1, that the existing Pak Kok Sewage Pumping Station (SPS) and Pillar Point Sewage Treatment Plant (STP) were able to handle the above discharge.
6.6.9 It is
therefore concluded that the wastewater generated during the operation of the
WFM can be effectively handled by the existing sewerage and treatment
facilities and will have no residual impact on the environment.
6.6.10 Runoff due to rainfall will be less polluted compared to other types of wastewater from the site, nevertheless it will add to the overall volume of wastewater generated at the WFM.
(i) Construction Phase
6.7.1 The following mitigation measures are proposed for the site during the construction phase:
· If possible, works should be planned to avoid rainy season so as to minimise the runoff and reduce the amount of soil that could be carried off site.
· The site should be kept clean and tidy to avoid construction material and waste being washed off site.
· Runoff should be directed to silt traps or sedimentation basin before reuse or discharge with the help of channels, earth bunds or sand bag barriers for suspended solids removal prior to its being discharged to storm water drain. The hoarding gaps should be tightly sealed to avoid the seepage of wastewater to the nullah and outside the site.
· Perimeter channels should be provided at site boundaries (where necessary) to intercept storm-water runoff from outside the site so that it does not wash across the site.
· Silt traps, sedimentation basins, channels and manholes should be regularly cleaned to remove the deposited silt and grit.
· Silt trap design should conform to the guidelines laid down in Appendix A1 of ProPECC PN 1/94[3].
· Temporarily exposed slope surfaces and construction material stockpiles should be covered with tarpaulin or similar fabric to prevent erosion.
· Wastewater likely to be contaminated with oil or grease should be passed through an oil separator or grease trap before entering the site drainage system.
· Water used in ground boring should as far as practicable be re-circulated after sedimentation. Thus, the consumption of water and the amount of effluent discharge from piling works can be reduced. The final discharge of the water should be via silt removal facilities.
· Vehicles and plants should be cleaned before they leave the site to prevent deposition of mud, debris, etc. on roads. Water collection tank(s) should be provided near the wheel washing facilities at the site entrance. Thus, the wastewater can be reused. The wash water should have silt and sand removed prior to discharge into storm drains.
· Wastewater from toilets, kitchen, etc. should be discharged into a foul sewer or a sewage treatment facility either directly or indirectly by means of pumping or other means considered appropriate by the Engineer. Alternatively, chemical toilets may be used for toilet wastewater and kitchen wastewater may be discharged into a foul sewer via a grease trap.
· Construction site discharges are controlled under the Water Pollution Control Ordinance (WPCO) and as such a discharge license will be needed for discharging the wastewater generated on site.
· ArchSD site supervisory staff will monitor the quality of water discharge to ensure compliance.
6.7.2 The following mitigation measures are proposed for the site during the operational phase:
· Toilet wastewater resulting from users of the fish market, community hall, etc. should be discharged into a foul sewer. No effluent discharge into the nullah will be allowed. The flush water storage tank will be properly designed so as to minimise the amount of water for each flush.
· Wastewater resulting from the cleansing of floors of the fish market and the refuse collection units (RCP) should be discharged into a foul sewer in order to avoid the direct discharge of wastewater into the nullah.
· Runoff due to rainfall will be considered for washing use after filtering if appropriate and other rainfall would be directed to storm drains.
6.8.1 The water quality of Castle Peak Bay is generally good. The construction activities at the site are of ordinary nature with no expected unusual water quality impact. The site discharge is subjected to control by the Water Pollution Control Ordinance (WPCO). A discharge license must be obtained before commencement of the work. The license will specify the allowable limits for the parameters of concern in the wastewater. Implementation of the suggested mitigation methods will help reduce the water quality impact to a minimal level.
1 Statement of Water Quality Objectives (North Western Water Control Zone): Chapter 358 Subsidiary Legislation, Section 2(3), Laws (Loose-Leaf Publication) Ordinance. (1990)
2 Technical Memorandum – Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters, Environmental Protection Department. (1991)
[3] Practice Note for Professional Persons, ProPECC PN 1/94, Professional Persons Environmental Consultative Committee, 1994